pe...@itl.co.il writes: >Dear Members, > >I am sure some of you can give me ideas on how to analyze a card >module which in four different occassions caused charring and >smoke within the equipment. In all cases, fire did not spread and >the charring occurred in all cases, arounf decoupling SMT >capacitors located between the Vcc and ground. > >The subject card sits in an industrial type computer within a GSM >equipment. The card is fed by 5 V and 12 Vdc from the computer >power supply and employs two SE:V interfaces. It employs a >piggy board with a receiver with direct connection to an external >outddor antenna. > >Any suggestions how this may have happened in the field, on four >different occassions and the charring/burning occured around the >decouplng capacitors. > >For this let us assume PWB is flame rated 94V-0 and is >Recognized. > Peter,
I have had several cases where something similar happened. In our case, the problem was with tantalum decoupling capacitors between voltage supplies and ground (both +5 and +/- 15 V). In one case one of the caps actually exploded and melted wiring harnesses at a customer's site. We recovered the product and analyzed the problem. We were unable to find a root cause at first. Then capacitors started exploding on the test floor at our factory. After much searching and testing we finally decided on two root causes. First was a 20 volt rating for a capacitor on the +15 volt rail. Should have been OK but we finally found surges to +30 volts during certain phases of product start-up. Second root cause appeared to be related to a certain manufacturer of caps. They had a higher failure rate than the other 4 authorized vendors. This problem came and went as Purchasing switched between suppliers to keep prices in check. Never did find what in our product caused this higher failure rate with this one vendor so, after burn-in testing of about 10,000 capacitors we finally removed that vendor from the vendor list for that particular part. The vendor still provides other parts that were not involved in this particular problem. Re-reading your note makes me think that the outdoor antenna may also play a role here in terms of lightning. Probably not connected but lightning does strange things to equipment. Any possibility for that? I hope this helps. Regards, Scott s_doug...@ecrm.com --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
